The absorption coefficient of electromagnetic radiation such as, for example, X-rays, in most materials strongly depends on the radiation's photon energy (E) and decreases approximately exponentially of the order E−3 when one neglects the K absorption edges, which may also be referred to as the radiation's absorption limit.
The contrast of X-ray images arises from absorption differences between the materials of the object being imaged, i.e. from differences between the materials' absorption coefficients. The lower the energy of the X-ray radiation engaging with the object, the higher is the contrast of the image thereof, provided that some of the low energy photons still permeate the object. Accordingly, an increase of the energy of the X-ray radiation results in a decrease of the image's contrast.
X-ray tubes as known in the art emit X-ray radiation of a relatively broad energy distribution due to the Bremsstrahlung (or “braking radiation”)-spectrum to which the X-ray radiation may be subjected to. By employing suitable filters, some of the spectrum can be filtered out. Such filtering however, is always at the expense of the radiation's flux density and may thus render such filtered X-ray radiation commercially inapplicable.
Detection of X-ray radiation is accomplished in a different way than detection of radiation of the visible spectrum, as silicon semiconductors can not be employed as detection means for X-ray radiations, since such radiation engaging with a silicon semiconductor based detection means would either pass through the silicon semiconductor or damage or disturb at least some of the components thereof. In the art, a way to detect X-ray radiation is to convert photons of the X-ray radiations into visible photons, which are detectable by commonly used photon detection devices based on silicon semiconductors. Conversion of photons of X-ray radiation into visible photons may be accomplished by employing a scintillator medium (e.g. a scintillator crystal), which absorbs photons of the X-ray radiation and emits in response energy in the form of visible photons. Accordingly, a photon detection device used in association with the scintillator medium has to be sensitive to radiation emitted by the same. Fortunately, the semiconductor silicon is actually sensitive in the frequency spectrum of the photons emitted by common scintillator media.
The energy of one photon of X-ray radiation can ionize several scintillator molecules. One photon of X-ray radiation may thus cause the scintillator medium to emit numerous packets of visible photons (e.g. hundreds or thousands of visible photons, whereby the number of visible photons emitted by the scintillator medium depends on the energy of the same photon of X-ray radiation. The scintillator medium is optically coupled to the photon detection device in a manner known in the art by, for example, employing a fiber plate that couples the photon detection device with the scintillator medium, or by abutting the scintillator medium to the photon detection device.
The main characteristics of the scintillator medium are its efficiency, the conversion ratio, decay time and the output wavelength. The efficiency is the probability of capturing an incoming photon of X-ray radiation and depends on the material but also on the thickness of the scintillator medium, i.e. a thicker layer will collect more of the photons. Efficiencies can rise, e.g., up to 95%. The conversion rate gives the amount of generated visible photons versus the energy of the photon. In a first approximation, the correlation is linear, i.e. when for example, a 25 keV X-ray photon may generate 500 visible photons; a 50 keV-photon may generate 1000 visible photons. The decay time tells how fast the excited molecules release their electrons and send out the visible photons. The shorter this decay time, the brighter is the light flash emitted by the scintillator medium and the more visible photons corresponding to X-ray radiation can be detected by the underlying photon detection device. A common decay time is for example 1 μs.
It should be noted that the term “visible photon”, “visible spectrum” and the like as used herein encompasses the spectrum of near-infrared light to soft-ultraviolet light (e.g. from 1000 nm to 200 nm).
Reference is now made to FIG. 1A. An X-ray sensor 100 used in the art comprises a scintillator medium 110, a substrate 130, a photon sensor unit 140 and an output pad 160. X-ray sensor optionally includes a spacer medium 120 which may be sandwiched between substrate 130 and Scintillator medium 110. Scintillator medium 110, which may be implemented by a suitable scintillator crystal, is adapted to convert X-ray radiation 10 engaging therewith into radiation of the visible spectrum, which is hereinafter referred to as “visible photons” 20. Photon sensor unit 140, which may be implemented by means of a photon-sensitive film or a semiconductor (e.g. a photodiode), is positioned relative to scintillator medium 110 to be able to detect at least some of visible photons 20. The higher the energy of X-ray radiation 10, the more visible photons 20 are generated by Scintillator medium 110 and the more visible photons 20 are therefore impinging on photon sensor unit 140 for each respective pixel.
If photon sensor unit 140 is implemented by means of a semiconductor, then photon sensor unit 140 may be operatively connected to an output unit 160 via an amplification path 150 comprising one or more amplifiers 151 Moreover, photons are converted into corresponding electronic signals by generating electron-hole pairs in the semiconductor and collected during a certain time frame (hereinafter referred to as “integration time”) onto a capacitance the respective charge carriers to generate a corresponding voltage or current for readout.
The integration time is set such that enough charge carriers are collected in and/or on the capacitance to obtain satisfying readout accuracy. However, the integration of charge carriers results in that the photons of radiation 10 are weighted in accordance to their energy. Accordingly, an increase in the energy of radiation 10 results in an increase in the weighting of corresponding visible photons 20. However, as outlined hereinabove, an increase in energy of X-ray radiation 10 above an energy-threshold where most of X-ray radiation 10 penetrates the object results in a decrease of the contrast of the object's image. Thus, employing high energy X-ray radiation 10 in order to obtain more visible photons 20 is detrimental to the desire of obtaining an image of high contrast as shown by Giersch et al. in “The influence of Energy Weighting on X-ray imaging Quality”, which is in incorporated by reference in its entirety herein and published in Nucl. Instr. Meth. A 531 (2004)64-74. To overcome this problem, Giersch et al. disclose a technique for weighing X-ray photons according to their respective energy. The technique improves the image quality (in terms of signal-to-noise ratio compared to integrating photon sensor units) up to a factor of 1.5. In the presence of scattered radiation the usability of lower photon energies is limited. For this case Giersch et al. derived an adapted weighting function, and further studied the influence of detector properties on the image quality improvement. The results of the study show that the energy bin size (i.e. number of energy channels) of the imaging system has a comparatively small impact on the benefit. Additionally, with optimized bin border positions, it is possible to achieve about 90% of maximum improvement with only three bins.
As a further alternative, X-Ray photons 10 may be counted instead of integrated. Counting x-Ray photons 10 implies that the X-Ray photons 10 are equally weighted, independent to their respective energy. To enable counting X-Ray photons 10, photon sensor unit 140 ought to be sensitive enough to detect two subsequent visible photon bursts 20 engaging with photon sensor unit 140 at a time interval that is close to the decay time of the scintillator, i.e., photon sensor unit 140 should have a detection resolution generally equal or shorter than said decay time. Various signal processing methods can be employed to increase sensitivity. Photon sensor unit 140 enabling the counting of X-Ray photons 10 generally employ a linear transfer function, whereby an electronic pulse resulting from a detection of at least one X-Ray photon 10 may be filtered by a resistor-capacitor (RC) filter and/or a capacitor-resistor (CR) filter to reduce noise incorporated in the electronic pulse.
A detailed description of such a detection means employing a linear transfer function is given by in “Development of a Radiation hard Pixel Analog Block for the CMS Vertex Detector and search for rare Decays at CMD-2” to Lechner, Dissertation at the Swiss Federal Institute of Technology, Zurich, October 1998, which is incorporated herein by reference in its entirety.
Additional reference is now made to FIG. 1B and FIG. 1C. Generally, only a few (hundreds to thousands) charge carriers, e.g. electrons, are collected onto capacitances respective to photon detection devices 180 and 190. These charge carriers will generate only a very small signal on the capacitance of the photodiode. For example, if a capacitance has 100 fF, 500 electrons will generate a signal of 0.8 mV. Therefore the signal has to be amplified for further processing. An integrating amplifier may serve the need of amplification. In an integrating amplifier, the charge is transferred from the large photodiode capacitance to a much smaller integrating capacitance in the feed-back path, e.g., 10 fF instead of 100 fF will thus generate a ten times larger output of 8 mV. As a consequence, an integrating amplifier produces larger output signals than the simple charge collected on the photodiode. If however the integrating amplifier just integrates the charge, it will saturate after a given number of events. The integrating amplifier should therefore operate in association with a discretely (FIG. 1B) or continuously (FIG. 1C) operating reset switch 182 and 192, respectively. Both switches have specific disadvantages: With the discretely operating reset switch, it must be ensured that the total charge collected during the integration time will not saturate the amplifier. On the other hand, the continuous reset needs a resistor in parallel with the capacitance; their RC-constant must be equal or even higher than the decay time of the scintillator medium otherwise the pulse will be suppressed already in this amplification stage. With an integration capacitance of for example, 10 fF, the resistor value has to be 100 MΩ. Realizing such a high resistance value in a Complementary Metal Oxide Semiconductor (CMOS) process is difficult, especially because the space is limited in the pixel. A high resistance value can be achieved by operating a MOS transistor in weak inversion. The actual impedance of such a resistor is however very sensitive to small production variations, which leads to sensitivity variations from pixel to pixel Photo Response Non Uniformity (PRNU). Such variations may be compensated by a calibration on the pixel level.
A further implementation of photon sensor unit 140 employing the technique of merely counting X-Ray photons 10 is disclosed by Liopart et al. in “Medipix2, a 64 k pixel read out chip with 55 μm square elements working in single photon counting mode”, Nuclear Science Symposium Conference Record, 2001 IEEE, Volume 3, Issue, 4-10 Nov. 2001 Page(s): 1484-1488 vol. 3, which is incorporated herein by reference in its entirety. The Medipix2 chip detector consist of 256*256 identical elements, each working in single photon counting mode for positive or negative input charge signals. Each pixel cell contains around 500 transistors and occupies a total surface area of 55 μm*55 μm. A 20 μm width octagonal opening connects the detector and the preamplifier input via bump-bonding. The preamplifier feedback provides compensation for detector leakage current on a pixel by pixel basis. Two identical pulse height discriminators are used to create a pulse if the preamplifier output falls within a defined energy window. These digital pulses are then counted with a 13-bit pseudo-random counter. The counter logic, based in a shift register, also behaves as the input/output register for the pixel. Each cell also has an 8-bit configuration register which allows masking, test-enabling and 3-bit individual threshold adjust for each discriminator. The chip can be configured in serial mode and read out either serially or in parallel. The chip is designed and manufactured in a 6-metal 0.25 μm CMOS technology.
As an alternative to integrating charge carriers, visible photons 20 can be converted directly into an output voltage by employing a transimpedance amplifier. The output voltage is dependent on the resistance in the feed-back path. More specifically, the dependency of an electronic current signal “Isig” to a current pulse “Ipulse” induced in photon sensor unit 140 due to the engagement of visible photons 10 therewith, may be expressed, for example, by the following equation:Isig=Ipulse·e−t/τ  (1)wherein, “τ” represents the time constant of the exponential decay of Ipulse.Correspondingly, if the maximum value of Ipulse equals for example, 80 pAmpere and the time constant τ equals, for example, 1 μs, then the equation becomes:Isig=80pAmpere·e−t/1 μs  (2)whereby an integration of the current pulse of, e.g., 80 pAmpere, over time yields a charge of 80 pAmpere*1 μs=0.08 fAs [femtoampere-seconds]=8*10−14 As [ampere seconds], which correspond to the charge of 500 electrons. Consequently, in order to generate a voltage signal of the same amplitude of 8 mV the resistance has to be 100 MΩ, which is way too high to be achievable in a pixel unit. The signal corresponding to the output voltage can be readout at any time but the signal is noisy due to the often small photon flux and the therefore small current flux. If the signal has a certain bandwidth, the signal-to-noise ratio can be increased by adding suitable filters, which reduce the out-of-band noise.
Another solution is to use the amplifier in open-loop configuration: The amplifier would directly amplify the signal on the photodiode, e.g., if the amplifier has a gain G of, for example, 100, and the capacitance of the photon sensor unit 140 equals, for example, 100 fF, then the voltage signal of the capacitance of the photo sensor would be amplified to manageable peak of 80 mV as outlined in the following equation:
                                                        q              ⁡                              [                fAs                ]                                                    C              ⁡                              [                fF                ]                                              |                      *            G                          =                  80          ⁢                                          ⁢          mV                                    (        3        )            
As a consequence, employing an amplifier in open-loop configuration seems to be the ideal solution. To facilitate this solution, two issues have to be addressed: 1) the photodiode again has to be properly, and 2) the open-loop amplifier needs a biasing so that the input range thereof is adapted to the photodiode resetting. However, biasing of the photodiode (e.g. resetting the charges thereof) is associated with the same problems as biasing the integrating amplifier, as outlined hereinabove with reference to FIG. 1B and FIG. 1C. Correspondingly, employing a discretely operating reset switch may cause saturation of the photodiode, whereas employing a continuously operating reset switch may necessitate employing a resistor having a relative high resistive value. Moreover, since resetting is not achieved by employing feedback from the amplifier itself, it may be difficult to reach an optimum working point for the circuit. For example, if the biasing is only 100 mV apart from the optimum point, then an amplifier having gain G of 100 would have an output of 10 V, and the photodiode would therefore already be saturated. An implementation of photon sensor unit 140 addressing both issues is disclosed by Krummenacher in “Pixel detectors with local intelligence: an IC designer point of view”, incorporated herein by reference in its entirety, published in Nuclear Instruments and Methods in Physics Research Section A, Volume 305, Issue 3, p. 527-532. Publication Date: 08/1991.
The time required for resetting the charges in photon sensor units 140 employing a resistive reset mechanism is schematically illustrated in FIG. 1D. The term linear also encompasses the term “substantially linear”.